The invention relates to a universal joint prosthesis with a cap with a calotte-shaped outer surface.
A hip joint prosthesis disclosed in U.S. Pat. No. 3,808,606 has a metallic spherical part which is secured on a metallic thorn which can be anchored on a femur, that is, a thigh bone. A disadvantage of this known prosthesis is that the entire spherical part, as well as a part of the neck of the femur, must be removed. Furthermore, a deep hole must be drilled in the femur to anchor the thorn. Consequently, the femur is considerably weakened due to the large amount of healthy bone material that must be removed. Since foreign substances can lead to undesired reactions, the insertion of a prosthesis with so much metal is disadvantageous. Another disadvantage of this prosthesis is that the thorn has a much greater bending resistance than the bone part into which it protrudes. In the case of bending stresses, the thorn is therefore much stiffer than the bone and the areas of the bone in the proximity of the thorn are subject to extremely great stresses in bending.
Prostheses are already known which use a calotte-shaped cap as a substitute for the spherical part of the femur. The interior of the cap is bounded by a smooth cylinder surface, apart from an existing narrow and shallow ring slot, and at the inner end by a hemispherical or radial, likewise smooth surface. For the insertion of such a cap, the spherical part of the femur is cut off only to the extend that a pivot complementary to the interior of the cap remains.
In order to make sure that such a cap-shaped prosthesis fits firmly on the femur, it must be secured with a binder, the so-called cement. As it is known, for example, from the publication "Histomorphological Investigations of Coxa Femoral Ends Following Double-Cup Arthroplasty According to Freemann", M. D. Cserhati, L. G. Oliveira, H. A. C. Jacob and A. Schreiber, Archines of Orthopaedic and Traumatic Surgery, 94, 233-240, 1979, this cement detaches in the course of time from the bony pivot and the cap becomes loose. Such prostheses, therefore, generally last only for a relatively short time.
The loosening of the prostheses is presumably caused, at least partly, by an unfavorable action of the cement on the spongiosa of the bone.
The axis of rotational symmetry of the bony pivot and of the cap in an upright standing patient with a prosthesis for a hip joint is inclined toward the vertical. On the other hand, the main stress is substantially vertical. Forces, therefore, appear naturally and quite frequently which form an angle with the rotational symmetry axis of the cap. Such forces are very likely another very important cause of the loosening of the cap because relatively great force components directed transverse to the rotational symmetry axis can then result, which extend with certain large relatively continuous sections of the inner cap surface more or less parallel or tangential to the inner cap surface. In these sections, there is only a minor transmission of forces between cap and femur, and the outer layers of the bony pivot are in addition stressed in shear. Furthermore, the cap and the bony pivot can then perform small movements relative to each other. If forces forming an angle with the rotational symmetry axis of the cup must thus be transmitted, this results in unfavorable and irregular stresses on the bony pivot.